1. Field of Invention
The present invention relates generally to four-dimensional (4D) computed tomography (CT) quality assurance (QA) equipment. More particularly the present invention relates to devices used to calibrate, confirm and test the accuracy of movement-correlated 4D CT target-locating systems.
2. Description of the Prior Art
The technologies of intensity-modulated radiotherapy (IMRT) have undergone rapid change. Four-dimensional CT acquisition—that is, CT acquisition of a moving three-dimensional target object (for example, a tumor)—is commercially available, and can provide important information on the shape and trajectory of a tumor and normal tissues. The primary advantage of four-dimensional imaging over light breathing helical scans is the reduction of motion artifacts during scanning that can significantly alter tumor appearance.
Precise knowledge and control of three-dimensional dose distribution in considered to be essential for a favorable therapeutic outcome. The ability to deliver highly conformal dose distributions through intensity-modulated radiotherapy has become common for sites such as head and neck and prostate. When the target moves due, for example, to respiration, precise delivery of dose becomes more challenging.
Artifacts due to motion (known as “temporal aliasing artifacts”) during tomographic scans have been appreciated for many years. Three-dimensional CT images are typically obtained by taking a series of adjacent image slices (or, alternatively, a continuous helix of images) of a subject who/that is typically placed on a platform (e.g., bed) that moves relative to the scanner, and are then digitally stitching (via computer software) the various image slices together. If the subject moves relative to the bed, as for example during breathing, while adjacent image slices are being scanned, the movement can result in temporal aliasing artifacts.
In order to minimize such temporal aliasing artifacts, motion-correlated CT systems have been proposed. Motion-correlated CT systems that acquire 3-dimensional image data are referred to herein as four-dimensional CT (“4D CT”) systems.
Respiration-correlated CT uses a surrogate signal, tracking movement such as of the abdominal surface, or of respiratory air flow, or of internal anatomy to provide a signal that permits re-sorting of the reconstructed image data, resulting in multiple coherent spatiotemporal data sets at different respiratory phases. The scan time for 4DCT with multislice scanners is on the order of a few minutes. In general, in order to re-sort and correlate the image data, each image slice is time- and/or position-stamped and each surrogate signal is time- and/or position-stamped. Computer software is then used to re-sort and correlate the various image slices into the proper sequences as dictated by the time- and/or position-stamps of the respective surrogate and the image slices. The output of this process is typically 10 CT volumes, each with a temporal resolution of approximately 1/10th of the respiratory period.
The ability of a 4D CT system to accurately re-sort and faithfully reconstruct three-dimensional data sets of a moving target volume within a subject depends heavily on how accurately and precisely the system can track the actual position in space of that target volume relative to the subject at all times during the scanning process.
There is a need, then, for a means to confirm, measure and calibrate the accuracy and precision with which a 4D CT system tracks the actual position in space of a scanned target volume.